Jędrzejczyk discovered Muay Thai when she was 16, while taking up fitness, which eventually led her to compete in national and European tournaments.[11][12] Jędrzejczyk competed in Muay Thai and kickboxing for 10 years, winning over 60 matches. Her amateur Muay Thai accomplishments include six IFMA world medals (five gold, one silver) and four IFMA Eurpopean championships with a record of 37 wins and 3 losses. During her professional career Jędrzejczyk won five world titles including the WKN World Championship, J Girls Championship, WBKF Championship, WKF European Championship, and the WMC Championship.[13][14] Her professional record is 27-3.

Jędrzejczyk next faced undefeated strawweight top contender Cláudia Gadelha on December 13, 2014, at UFC on Fox 13.[22] She won the fight via split decision. The judges' decision was met with much controversy as the majority of the MMA media scored the bout in favor of Gadelha.[23]

Jędrzejczyk's first title defense was against Jessica Penne on June 20, 2015, at UFC Fight Night: Jędrzejczyk vs. Penne in Berlin, Germany. She successfully defended the title against Penne, defeating her in the third round via TKO following a flurry of punches and a knee against the cage. Both participants were awarded Fight of the Night honors.[27][28] Jędrzejczyk later underwent surgery to repair a fractured thumb incurred during the fight.[29]

In her second title defense, Jędrzejczyk faced Valérie Létourneau on November 15, 2015, in the co-main event at UFC 193. She won the fight by unanimous decision to retain her title.[30] The 220 significant strikes landed by Jędrzejczyk were the most ever in a championship fight.[31] 70 leg kicks were landed by Jędrzejczyk, the most in UFC history. The previous record was 68, set by Carlos Condit against Nick Diaz at UFC 143.[31]

Jędrzejczyk faced Jéssica Andrade on May 13, 2017, at UFC 211. She won the fight via unanimous decision, and successfully defended her title for the fifth consecutive time.[36] Jędrzejczyk landed the most significant strikes in UFC championship history (225), had the highest significant strike differential in UFC championship history (142), and broke her previous record for most leg kicks thrown in a single fight (75).[37]

For her sixth title defense, Jędrzejczyk faced Rose Namajunas at UFC 217 at Madison Square Garden in New York on November 4, 2017. She was confident going into the fight and was knocked down early in the first round after Namajunas threw a left hook that knocked Jędrzejczyk down. After standing back up, a few seconds later, Jędrzejczyk went on to lose her title after she was caught with a left hook on the chin by Namajunas and face planted on the floor. Rose proceeded to deliver strikes to her head until Joanna tapped due to strikes. This was Jędrzejczyk's first loss in her professional MMA career.[38]

On September 20, 2018, it was announced that Jędrzejczyk was expected to return to Flyweight to face Valentina Shevchenko on December 8, 2018, at UFC 231 for the vacant UFC Women's Flyweight Championship.[43] Due to UFC 230 being in need of a main event, Shevchenko was booked against Sijara Eubanks.[44] After the UFC announced Cormier vs. Lewis on October 9, 2018, UFC confirmed the bout between Shevchenko and Eubanks was canceled and that Jedrzejczyk would again fight for the title against Shevchenko in the co-main event at UFC 231.[45] Jedrzejczyk lost the fight via a unanimous decision.[46]

A decorated kickboxer, Jędrzejczyk is known for precise and calculated striking.[47] She employs a sprawl-and-brawl strategy in MMA fighting.[48][49] While standing with opponents, she constantly varies between high punches and body shots, also utilizing knees, elbows, and high front kicks.[50][51] She also uses a flurry of strikes just to confuse the opponent before landing a solid strike, much akin to Chuck Liddell.[52]SportsJoe.ie MMA journalist Darragh Murphy summed up her fighting style: "Her technique has the right combination of sublimity and relentlessness, expressing one's typically violent art through beauty, the perfect storm."[53]

While Jędrzejczyk is soft-spoken and has a quirky and endearing personality with fans,[54][55][56][57][58] she often employs the use of trash-talk and bullying against her opponents before fights,[59][60][61] and her fights have been known to be violent and bloody.[60][62][63] UFC president Dana White has praised her stating "She is so bad ass... pound-for-pound [she is the] best female fighter on Earth... Her killer instinct is ridiculous."[64] MMA media have called Jędrzejczyk "must-see TV."[65][66][67][68]

Jędrzejczyk has been praised for her elite level sprawl-and-brawl strategy implementation. Fightland striking analyst Jack Slack has written that "she's being likened to Chuck Liddell, and that is a pretty solid way to describe her...while she has an undoubtedly better technical striking game than Liddell".[69]BloodyElbow.com striking analyst Connor Ruebusch has written "Distance, angles, and damage. These are the key ingredients of successful takedown defense, and the elements that make Joanna Jędrzejczyk and José Aldo such masterful strikers."[49]The Washington Post striking analyst Patrick Wyman has written "Sound fundamentals fuel the best fighters in the world, and nobody has better mastered the bread and butter of footwork and distance management than Aldo and Jedrzejczyk".[70]Foxsports.com striking analyst Jason Parillo has stated "Joanna's MMA boxing is remarkable. It's second to none".[71]